Field
The present specification generally relates to borosilicate glasses and, more specifically, to borosilicate glasses having relatively low alkali content.
Technical Background
Glass substrates are commonly employed in a variety of electronics applications including substrates used in LED and LCD displays, touch screen applications and the like. Chemical strengthening of glass substrates used in such applications can be achieved by an ion exchange process in which smaller alkali metal ions in the glass are exchanged for larger alkali metal ions present in a molten salt bath in which the glass is placed. The amenability of the glass to be strengthened by ion exchange (i.e., its “ion exchangeability”) is due, at least in part, to the mobility of alkali ions in the glass. That is, the more mobile the alkali metal ions are in the glass, the more amenable the glass is to strengthening by ion exchange.
While the mobility of alkali metal ions in the glass may facilitate chemical strengthening, high alkali metal ion mobility is not always a desired characteristic in certain applications. For example, some glass substrates may be utilized for display device applications in which a plurality of thin film transistors (TFTs) is deposited on the surface of the glass substrate. When the glass substrate contains highly mobile alkali metal ions, the alkali metal ions may migrate into the TFT material and “poison” the TFTs, rendering them inoperable. Accordingly, for some applications, the amount of alkali metal ions in the glass is reduced or eliminated.
However, eliminating the alkali metal ions from a glass also has negative implications. For example, glasses are typically melted by flowing electric current through the glass. When the highly mobile alkali metal ions are reduced or removed from the glass, the high temperature resistivity of the glass increases significantly along with the voltages necessary to melt the glass which, in turn, causes the refractory materials containing the glass to more rapidly degrade or even fail.
Accordingly, a need exists for alternative glasses which have relatively low high temperature resistivity to aid in melting and relatively high low temperature resistivity to reduce the mobility of alkali metal ions in the glass.